Nonremoval technologies are those that involve the remediation of contaminated sediments in situ (i.e., in place). Nonremoval technologies for contaminated sediments include in situ capping, in situ containment, and in situ treatment.

Nonremoval technologies are single-component remedial alternatives. They do not require sediment removal, transport, or pretreatment. As a result, nonremoval technologies are often less complex and have lower costs than multicomponent alternatives (e.g., combinations of removal, transport, treatment, and disposal). In some cases (e.g., in situ treatment), nonremoval technologies may be similar to the treatment and disposal technologies used with dredged sediments.

This chapter provides descriptions of sediment remediation technologies that have been demonstrated, designed, or considered for application in situ. Discussions of the factors used to select from the available technology types and techniques for estimating costs and contaminant losses are also provided.

In situ capping is the placement of a covering or cap over an in situ deposit of contaminated sediment. The cap may be constructed of clean sediments, sand, or gravel, or may involve a more complex design using geotextiles, liners, and multiple layers. An annotated bibliography prepared for the Canadian Cleanup Fund (Zeman et al. 1992) summarizes most of the capping projects and studies that have been completed to date.

Capping is also a viable alternative for disposal of contaminated sediments that have been dredged and placed in another aquatic location (this type of capping is discussed in Chapter 8). Much of the technical information and guidance provided herein has been adapted from that developed for dredged material capping in ocean waters. The guidance provided in this section focuses on in situ capping of contaminated sediments in riverine and sheltered harbor environments such as those commonly found in the Great Lakes region.

A limited number of in situ capping operations have been accomplished in recent years under varying site conditions. In situ capping has been applied in riverine, nearshore, and estuarine settings. Conventional dredging and construction equipment and techniques can be used for in situ capping projects, but these practices must be precisely controlled. The success of projects to date and available monitoring data at several sites indicate that in situ capping may be an effective technique for long-term containment of contaminants.

In situ capping of contaminated sediments with sand has been demonstrated at a number of sites in Japan (Zeman et al. 1992). Demonstration projects conducted at Hiroshima Bay evaluated various types of placement equipment. More recent studies have examined the efficiency of sand caps in reducing the diffusion of nutrients.

At the Denny Way project in Puget Sound, a layer of sandy sediment was spread over a contaminated nearshore area, with water depths of 6 18 m, using bottom-dump barges with provisions for controlled opening and movement of the barges (Sumeri 1989). This was accomplished by slowly opening the conventional split-hull barge over a time frame of 30 60 minutes, allowing the gradual release of the material in a sprinkling manner. A tug was used to slowly move the barge laterally during the release, and the material was spread in a thin layer over the desired area.

At the Simpson-Tacoma Kraft mill project in Puget Sound, an in situ capping project involved spreading hydraulically dredged sediment with surface discharge through a spreading device (Sumeri 1989). Hydraulic placement is well-suited to placement of thin layers over large surface areas. Specialized equipment and placement techniques developed for dredged material capping and in situ capping are shown in Table 3-1 (Palermo 1991b).

In situ capping using an armoring layer has also been demonstrated at a Superfund site in Sheboygan Falls, Wisconsin. This project involved placement of a composite cap, with layers of gravel and geotextile, to cover PCB-contaminated sediments in the shallow water (<1.5 m) and floodway of the Sheboygan River. The cap was placed using land-based construction equipment and manual labor. A typical cross section of the in situ cap for this project is shown in Figure 3-1.

A variation of in situ capping would involve the removal of contaminated sediments to some depth, followed by capping the remaining sediments in place. This method is suitable when capping alone is not feasible because of hydraulic or navigation restrictions on the waterway depth. It may also be used where it is desirable to leave the deeper, more contaminated sediments capped in place (vertical stratification of sediment contaminants is common in many Great Lakes tributaries).

While in situ capping isolates the contaminated sediments from the water column immediately above the sediments, in situ containment involves the complete isolation of a portion of the waterway. Physical barriers used to isolate a portion of a waterway include sheetpile, cofferdams, and stone or earthen dikes. The isolated area can be used for the disposal of other contaminated sediments, treatment residues, or other fill material. The area may have to be modified to prevent contaminant migration (e.g., slurry walls, cap and cover).

Perhaps the largest sediment remediation project undertaken to date has been at Minamata Bay, Japan, where 58 hectares of the bay with the highest levels of mercury-contaminated sediments was isolated using cofferdams, and 1.5 million m[3] of contaminated sediments from other areas of the bay were hydraulically dredged and placed into the enclosed area (Hosokawa 1993). The contaminated sediments were capped with volcanic ash, sand, and geotextile, and the area has been filled to grade.

On a far smaller scale, remediation at the Waukegan Harbor Superfund site included the isolation of a boat slip containing the highest levels of PCB-contaminated sediments. The slip was isolated using a double bentonite-filled sheetpile cutoff wall across the open end and a bentonite slurry wall around the landward perimeter. About 15,000 m[3] of contaminated sediment was hydraulically dredged from other areas of the harbor, placed into the isolated slip, and capped with clay and topsoil. A series of drawdown wells were installed around the perimeter of the isolated slip, and will be operated indefinitely to maintain an inward hydraulic gradient.

Some treatment technologies have been developed specifically for in situ application, while others have been adapted from ex situ treatment applications, including some of the technologies discussed in Chapter 7, Treatment Technologies. Most in situ treatment technologies could also be applied to sediments that have been dredged and placed in a disposal area.

In situ treatment has several limitations. One such limitation is the lack of process control. Process control is contingent upon effectively monitoring conditions at the site, typically by performing sampling and analysis at appropriate frequencies, before and after treatment. The efficacy of in situ treatment of sediments is difficult to determine because of the nonhomogeneous distribution of contaminants, sediment physical properties, and treatment chemicals. One of the limitations of in situ treatment is the difficulty in ensuring uniform dosages of chemical reagents or additives throughout the sediments to be treated. Areas of sediment within the site may receive varying levels of treatment, with some areas of sediment being untreated while others are overtreated relative to the intended treatment goal. In situ treatment may be less cost effective than ex situ treatment when these factors are considered.

Among the most significant limitations to in situ treatment is the impact of the process on the water column. Processes that would release contaminants, reagents, or heat, or produce other negative impacts on the overlying water column, are not likely to be acceptable for in situ sediment remediation. A suitable in situ treatment technology is, in most cases, one that can be applied with minimal disturbance of the sediment-water interface or one in which the process is physically isolated from the water column. There are two general methods of applying in situ treatment that address this limitation: surface application and isolation of the sediments prior to treatment. Several types of treatment processes might be used within these applications.

Surface application is the introduction of one or more materials (e.g., reagents, additives, nutrients) onto the sediments by spreading and settling, or injecting them into the sediments through tubes, pipes, or other devices. Researchers at the Canadian National Water Research Institute have developed and demonstrated equipment that is capable of injecting solutions of oxidizing chemicals into uncompacted sediments at a controlled rate (Murphy et al. 1993). A schematic of this apparatus is shown in Figure 3-2.

The second method for applying sediment treatment in place is by isolating the sediment from the surrounding environment. This method allows the use of reagents or process conditions that might otherwise cause deleterious effects to the waterway. Various types of equipment might be used for isolating the sediments, including a caisson, sheetpile cell, tube, or box. A hypothetical application using a sheetpile caisson is shown in Figure 3-3. Within the enclosing caisson, the water may be removed or left behind (if needed to support the process). One proprietary system (MecTool, Millgard Environmental Corp.) uses a bladder to isolate the sediments (and the treatment process) from the overlying water. Within the enclosed caisson, sediments can be mixed and treatment reagents can be added. After the treatment is completed, the caisson can be removed and reset at an adjacent area.

Three types of sediment treatment technologies that have been demonstrated or at least considered for in situ application will be discussed below: chemical, biological, and immobilization.

In situ Chemical Treatment
Sediments in lakes and reservoirs have been treated in situ to control eutrophication or other conditions (USEPA 1990i). Aluminum sulfate (alum) has been used to control the release of phosphorus from bottom sediments and thereby limit algal growth (Kennedy and Cooke 1982). The alum is typically spread over a large area of the lake, and allowed to settle through the water column and deposit on the sediment surface. Alum treatment is recommended for lake restoration in well-buffered, hard-water lakes (USEPA 1990i).

The injection of calcium nitrate into sediments to promote the oxidation of organic matter has been demonstrated in conjunction with lime and ferric chloride additions to promote denitrification and phosphorus precipitation (USEPA 1990i). Calcium nitrate injection is discussed below as part of a bioremediation application.

A detailed discussion of treatment technologies for toxic contaminants is provided in Chapter 7. Perhaps because of the limitations associated with in situ treatment, development in this area of treatment has been limited.

In situ Biological Treatment
Effective in situ bioremediation of fine-grained, saturated soils and sediments (as opposed to more porous groundwater aquifers or soils within the vadose zone) poses a major challenge. While delivery and transport of nutrient and electron acceptor amendments to and through groundwater aquifers is a demonstrated technology, movement of these materials through fine-grained sediments is difficult.

Contaminated sediments removed from the Sheboygan River Superfund site have been evaluated for biodegradation of PCBs in a confined treatment facility (CTF). These experiments as well as efforts to measure PCB dechlorination in sediments capped in situ in the Sheboygan River have been inconclusive as of early 1994.

A form of bioremediation has been demonstrated on PAH-contaminated sediments in Hamilton Harbor, Ontario (Murphy et al. 1993). Dissolved calcium nitrate was injected into sediments over 1.4 hectares using the system shown in Figure 3-2. The chemical injection oxidized about 80 percent of the hydrogen sulfide and stimulated the subsequent biodegradation of low molecular weight organic compounds (79-percent reduction). More moderate reductions in PAHs (25 percent) were shown.

In situ Immobilization
Immobilization alters the sediment's physical and/or chemical characteristics to reduce the potential for contaminants to be released from the sediment to the surrounding environment (Myers and Zappi 1989). The principal environmental pathway affected by in situ immobilization for sediments is leaching of contaminants from the treated sediment to groundwater and/or surface water. Solidification/stabilization is a commonly used term that covers the immobilization technologies discussed herein.

Binders used to immobilize contaminants in sediment or soils include cements, pozzolans, and thermoplastics (Cullinane et al. 1986b). Many commercially available processes add proprietary reagents to the basic solidification process to improve effectiveness of the overall process or to target specific contaminants. The effectiveness of an immobilization process for a particular sediment is difficult to predict and can only be evaluated by laboratory tests conducted with that sediment.

Ex situ solidification/stabilization processes are readily implemented using conventional mixing equipment. However, injection of a reagent to achieve a complete and uniform mix with in situ sediments is considerably more difficult and has not been demonstrated on a large scale. Reagents for the solidification process can be injected into the sediment in a liquid or slurry form. Porous tubes are sometimes used to distribute the reagents to the required depth. Available commercial equipment includes a hollow drill with an injection point at the bottom of the shaft. The drill is advanced into the sediment to the desired depth. The chemical additive is then injected at low pressure to prevent excessive spreading and is blended with the sediment as the drill rotates. The treated sediment forms a solid vertical column. These solidified columns are overlapped by subsequent borings to ensure sufficient coverage of the area (USEPA 1990e). In situ solidification/stabilization has been demonstrated in sediments at Manitowoc Harbor in Wisconsin, where a cement/fly ash slurry was injected through a hollow-stem kelly bar using a proprietary mixing tool (MecTool) and slurry injector. This process formed treated vertical columns 6 ft (1.8 m) in diameter to a depth of 6 m below the river bed, using a 6x25-ft (1.8x7.6-m) steel cylinder placed 1.5 m into the sediments in 6 m of water (similar to the setup shown in Figure 3-3). This demonstration experienced difficulties in solidification of some sediments and management of liberated pore water (Fitzpatrick 1994).

The nonremoval technologies discussed in this section represent single-component remedial alternatives, and are not as comparable as different technology types or process options of a multicomponent alternative (e.g., different types of dredges). Most nonremoval technologies are in the development stage and have only been applied at a small scale at a limited number of sites. As a result, guidance on their feasibility, design, and implementation is very limited. Factors for selecting nonremoval technologies, shown in Table 3-2, are not intended for comparison purposes, but to screen these technologies for overall feasibility at a particular project site.

The primary technical considerations that affect the feasibility of in situ capping are the physical and hydraulic characteristics and the existing and future uses of the waterway. The suitability of in situ capping to a contaminated sediment site is less affected by the type or level of contaminants present, because it physically isolates the sediments and their associated contaminants.

The ideal area for in situ capping would be sheltered and not exposed to high erosive forces, such as currents, waves, or navigation propeller wash, or to upwelling from groundwater. Depending on the erosive forces present at a site, an in situ cap may have to be armored with stone or other material to keep the cap intact. Areas on five tributaries of the Great Lakes were examined under the ARCS Program in developing guidance on the hydraulic design of in situ caps (Maynord and Oswalt 1993). River currents were the dominant erosive force in only one of five areas. The scour caused by navigation (recreational as well as commercial) was the dominant force in the other areas studied. The potential scour caused by large commercial vessels would necessitate very large armor stone, making in situ capping difficult in or near most active navigation channels (Environmental Laboratory 1987; Maynord and Oswalt 1993).

For some waterways, in situ capping may not be consistent with local or regional plans for waterway use. For example, if a reach of a river with contaminated sediment deposits is already shallow, an in situ cap may further reduce water depths to levels that are not safe for existing and planned recreational boating. Removal of some contaminated sediments and in situ capping for the remaining portion may be an option in this case. In all cases, the construction of an in situ cap represents a deliberate modification to the morphology of the bottom of a waterway. Future uses of the waterway may be limited by this modification.

Design Process for In situ Capping
Capping is a dredged material disposal technology that has been used by the Corps for over 10 years (discussed in detail in Chapter 8). Although there are many differences between in situ capping and dredged material capping, some of the design guidance for this disposal technology (Palermo et al., in prep.) is appropriate to in situ capping and is presented herein.

An in situ capping operation should be treated as an engineering project with carefully considered design, construction, and monitoring to ensure that the design is adequate. The basic criterion for a successful in situ capping operation is simply that the cap required to isolate the contaminated material from the environment be successfully placed and maintained. The elements of in situ capping design are listed in Table 3-3. The design considerations for in situ capping include selection and evaluation of capping materials, cap thickness, equipment and placement techniques for the cap, cap stability, and monitoring.

Data Collection--A variety of information about the project site and sediments is needed to prepare an in situ capping design. The areal extent and thickness of the contaminated sediment deposit should be defined by surveys of the area. The site conditions should also be defined to include bathymetry, currents, water depths, bottom sediment characteristics, type and draft of adjacent navigation, and flood flow. The contaminated sediment deposit to be capped must be characterized for both physical and chemical parameters.

Physical characteristics are important in determining the suitability of placement of various capping materials. In situ density (or solids content), plasticity, shear strength, consolidation, and grain size distribution are needed for evaluations of resistance to displacement.

Capping Material--Various types of capping material may be used for in situ capping. If available, dredged sediment from navigation projects can be used. This option could be considered a beneficial use of material that might otherwise require disposal elsewhere. In other cases, removal of bottom sediments from areas adjacent to the capping site may be considered. Material other than sediments is also an option for the cap, such as clean fill from offsite sources, geotextiles, stone/gravel, and grout mattresses. In general, sandy sediments are suitable for use as a cap at sites with relatively low erosive energy, while armoring materials may be required at sites with high erosive energy.

Cap Thickness--The cap must be designed to chemically and biologically isolate the contaminated material from the aquatic environment. For sediment caps, the determination of the minimum required cap thickness is dependent on the physical and chemical properties of the contaminated and capping sediments, the potential for bioturbation of the cap by aquatic organisms, the potential for consolidation and erosion of the cap material, and the type(s) of cap materials used. Laboratory tests have been developed to determine the thickness of a capping sediment required to chemically isolate a contaminated sediment from the overlying water column (Sturgis and Gunnison 1988). The minimum required cap thickness for chemical isolation is on the order of 30 cm for most sediments tested to date. Bioturbation depths are highly variable; however, in Great Lakes sediments they are typically on the order of 10 cm. The minimum thickness of capping sediment for most projects will therefore be determined by constructability constraints. Conventional equipment and placement accuracies will dictate minimum cap thicknesses of 50-60 cm.

Geotextiles may be incorporated into in situ caps for a number of purposes, including: stabilizing the cap, promoting uniform consolidation, and reducing erosion of the granular capping materials.

Geotextiles and synthetic liners might also be incorporated into the cap design to limit bioturbation and provide contaminant isolation (Palermo and Reible, in prep.). A geotextile was incorporated into the cap used at the Sheboygan River (Figure 3-1), and a geotextile has been used as part of a contaminated sediment cap in Sorfjord, Norway (Zeman 1993).

An armoring layer for resistance to erosion can also be considered in the cap design (Environmental Laboratory 1987; Maynord and Oswalt 1993). For caps composed of armoring layers, the chemical isolation would be dependent on a filter, while the armor layer would normally prevent any disturbance of the cap by bioturbation and would be designed to resist erosion. Consideration must be given, however, of the potential attraction to benthic species of the new surface provided by the armoring layer.

Equipment and Placement Techniques--For sediment caps, the major consideration in the selection of equipment and placement of the cap is the need for controlled, accurate placement of the capping material (and the associated density and rate of application of the capping material). In general, the capping material should be placed so that it accumulates in a layer covering the contaminated material. The use of equipment or placement rates that would result in the capping material displacing or mixing with the contaminated material must be avoided.

Pipeline and barge placement of dredged material for in situ capping projects is appropriate in more open areas such as harbors or wide rivers. Specialized equipment and placement techniques developed for dredged material capping that might be considered for in situ capping are shown in Table 3-1 (Palermo 1991b). In constricted areas, narrow channels, or shallow nearshore areas, conventional land-based construction equipment may be considered.

Once the equipment and placement techniques for the cap are selected, the need for land-based surveys or navigation and positioning equipment and controls can be addressed. The survey or navigation controls must be adequate to ensure that the cap can be placed (whether by land-based equipment, bargeload, hopperload, or pipeline) at the desired location in a consistently accurate manner.

Monitoring--A monitoring program should be considered as a part of any capping project design (Palermo et al. 1992). The main objectives of monitoring for in situ capping would normally be to ensure that the cap is placed as intended and the required capping thickness is maintained, and that the cap is effective in isolating the contaminated material from the environment.

Intensive monitoring is necessary at capping sites during and immediately after construction, followed by long-term monitoring at less frequent intervals. Based on Corps experience at dredged material capping sites in New England, long-term monitoring should include bathymetric surveys, camera profiles, and occasional core samples (Fredette 1993). In addition to physical and chemical monitoring, biological monitoring may be conducted to track recolonization of benthos and evaluate contaminant migration. In all cases, the objectives of the monitoring effort and any remedial actions to be considered as a result of the monitoring should be clearly defined as a part of the overall project design.

The technical feasibility of using in situ containment is determined primarily by the physical conditions of the site. Areas that may be suitable for in situ containment include backwater areas, slips, turning basins, and some wide areas of rivers. Areas within active navigation channels are generally not suitable.

The primary factors limiting the feasibility of in situ containment are the potential impacts of the new fill on flow patterns, flooding, navigation, and habitat. Slips and turning basins are especially well suited, because they only need to be isolated at one end and can generally be filled without reducing the hydraulic capacity of the adjacent river channel.

In situ containment will require structural measures and environmental controls to isolate the containment area from the adjacent waterway and prevent unacceptable contaminant migration. Testing and evaluation to determine the appropriate controls is discussed in Chapter 8, Disposal Technologies.

It may also be possible to completely reroute waterways with contaminated sediments. The waterway can then be dewatered, and the sediments removed, treated in place, or confined in place. This is an extreme measure and is only likely to be feasible for small waterways with limited flows.

There are three primary considerations in evaluating the suitability of in situ treatment. The first consideration is whether the treatment process can work effectively under the physical conditions of in situ sediments (i.e., saturated, anaerobic, and ambient temperatures). Treatment technologies that require greatly different conditions are less likely to be feasible for in situ application. Bench-scale testing of proposed treatment technologies should be performed to determine if the process can operate effectively under in situ conditions. Treatment technology testing is discussed further in Chapter 7.

The second consideration is the level of control needed to apply the treatment technology. Some technologies require well-mixed systems in order for reagents and sediment contaminants to react effectively. Specialized equipment may be needed to introduce reagents and manipulate the sediments. The development of such equipment may require pilot- or full-scale testing. Technologies that require greater levels of sediment manipulation are less likely to be feasible for in situ applications.

The third consideration is the environmental impact on the water column and aquatic environment. Suitable treatment technologies must be able to operate without dispersing the sediments, releasing toxic reagents or contaminants, or creating conditions more harmful to aquatic life than already exist. Examples of specialized equipment to isolate the treatment process from the water column are shown in Figures 3-2 and 3-3.

There is little detailed cost information in the literature about in situ remediation technologies, even for those that have been implemented. Available information about applications that have been implemented or proposed is summarized in Table 3-4 [part i] [part ii].

Capital costs for in situ capping include costs of capping materials, construction equipment, and labor. These costs will be influenced by the complexity of the cap design, accessibility of the capping site, water depth, and other factors. If clean dredged material (e.g., from a navigation project) can be used in a capping application, capital costs could be greatly reduced.

Operation and maintenance costs include monitoring and periodic cap replenishment. Based on the experience of the Corps' New England Division with dredged material capping, the costs for a routine long-term monitoring cycle (bathymetric surveys and camera profile) are about $30,000 (Fredette 1993). This basic physical monitoring cycle is conducted every 2 3 years. More extensive monitoring (including sediment cores and biological monitoring) is conducted on a less frequent cycle.

Capital costs for in situ containment include the materials, equipment, and labor needed to construct the caisson, bulkhead, dike, or revetment, which isolates a portion of the waterway. Typical costs for marine sheetpile construction in the Great Lakes are $12-17/ft[2] ($130-180/m[2]) (Wong 1994). Additional capital costs may be related to the filling of the enclosed area with contaminated sediments (or other materials) and the environmental controls necessary for the enclosed site. These dredging and confined disposal costs are discussed in Chapter 4 (Removal Technologies) and Chapter 8 (Disposal Technologies). Operation and maintenance costs for in situ containment include monitoring and routine maintenance of the structure.

Capital costs for in situ treatment include the costs of equipment, materials, reagents, and labor necessary to treat the sediments. The development and fabrication costs for the application equipment may be significant. A substantial amount of development cost may also be required for the treatment process itself, if it has not been applied in situ.

The loss of contaminants from sediments in situ is a primary rationale for remediation. The amounts of sediment contaminants lost during and after remediation need to be estimated to determine the benefits of remediation and to evaluate the impacts of remedial alternatives. The mechanisms for contaminant losses associated with nonremoval technologies are summarized in Table 3-5.

Estimating contaminant losses for nonremoval technologies is difficult because of the lack of field monitoring data and standard procedures for assessing nonremoval technologies. Predictive models based on diffusion are conceptually applicable to most nonremoval technologies. The seepage/leaching losses from an enclosed area constructed for in situ containment can be estimated using the predictive models developed for CDFs (see Chapter 8, Disposal Technologies). However, predictive techniques are not available that account for any of the other mechanisms of contaminant loss associated with nonremoval technologies.

Contaminant losses during placement of a cap, construction of an isolation wall, or the injection of reagents or additives for chemical treatment or immobilization can result in highly localized, but transient disturbances of contaminated sediment. For example, during in situ immobilization, contaminant losses occur at the point of additive injection, and injection-related losses last only as long as additives are being injected. These highly localized and transient disturbances can be as important as long-term diffusion losses. At present, highly localized, transient contaminant losses associated with the implementation of nonremoval technologies cannot be predicted. In addition, nonremoval technologies involving several processing steps, especially those involving mixing of the contaminated sediments, will have more contaminant loss mechanisms to consider than simpler nonremoval technologies, such as in situ capping.

Once the implementation phase of a nonremoval technology is completed, diffusion is the major contaminant loss pathway. Advection, bioturbation, and biodegradation can also be important in some cases, but can be avoided by careful planning, design, preproject testing, and monitoring. For example, sites with significant groundwater movement through the sediment (and associated significant contaminant losses) are not good candidates for nonremoval technologies. Controls for bioturbation should be part of engineering design, and the potential for biodegradation of solidified matrices following immobilization processing should be evaluated in a laboratory testing phase.

The application of diffusion models to certain nonremoval technologies, such as in situ capping and in situ immobilization, is better established than the application of these models to other nonremoval technologies, such as in situ chemical treatment. The diffusion models are described in detail in Myers et al. (in prep.). Cap thickness, sorption properties of the cap, contaminant chemical/physical property data, and sediment chemical/physical property data are variables needed to evaluate in situ capping effectiveness. For in situ immobilization, process-specific physical and chemical data are needed, including bulk density, contaminant concentration after processing, effective diffusion coefficients, and durability data. For other nonremoval technologies, there may be additional information needs.